Cut the cucumbers into thin 1⁄8-inch slices. Put the sliced cucumbers in a bowl and toss them with the kosher salt. Let them sit at room temperature for 20 minutes; the salt will draw some of the water from the cucumber.

While the cucumber is wilting, peel and grate the ginger; rinse and chop the ogo.

Mix the rice vinegar, sugar, mirin, and grated ginger in a bowl or container.

When the cucumbers are wilted, put them in a colander and rinse under running water to remove excess salt.

Squeeze them lightly with your hands or roll them in a towel to remove even more water. Add the drained cucumber and chopped ogo to the vinegar mixture and stir to coat.

Let the namasu marinate in the refrigerator for at least 30 minutes. Just before serving, add the king crab meat.